1. Field of the Invention
The present invention relates generally to the field of high voltage component housing systems, and more specifically to the encapsulation of high voltage vacuum devices within housings in an improved manner.
2. Description of the Related Art
Many high voltage devices, such as vacuum tubes, high voltage resistors, and high voltage capacitors, that operate at high voltage in high voltage component systems require specialized housing methods to provide the necessary high voltage insulation of the high voltage devices, the interface to power supplies, and/or to mounting surfaces for subsequent use in systems. Additionally, such housing systems must often remove heat due to high temperatures generated by operation of the high voltage device. For instance, conventional x-ray tubes, such as those illustrated in U.S. Pat. Nos. 5,751,784 and 5,563,923, typically include a sealed glass or ceramic vacuum tube containing the x-ray generating means, which requires appropriate insulating, cooling and shielding.
To achieve the above-mentioned insulation and thermal handling objectives, one type of conventional housing system used for vacuum tubes involves filling the housing with high dielectric strength, high purity oil into which the vacuum tube is submerged. For instance, for vacuum x-ray tubes, Diala or another comparable high dielectric strength oil has been filled into an outer housing, and the vacuum x-ray tube is submerged in the oil. For such vacuum x-ray tube assemblies, a diaphragm also is typically placed between the exposed surface of the oil and the air outside the outer housing for sealing purposes and so that the oil can expand from heating without applying stress to the vacuum x-ray tube.
Oil systems of this type for vacuum tubes have been popular due to the relative simplicity in implementation. If proper cleanliness, elimination of air voids, and room for expansion of the oil during heating is provided, the oil systems can provide excellent high voltage insulation with good heat coupling to the housing. Oil systems also permit easy removal of the vacuum tube, if necessary due to manufacturing, test or operational problems, by simply draining the oil and replacing the vacuum tube with another, followed by refilling the oil.
However, drawbacks are associated with the oil systems in practice. The use of the liquid oil requires care to avoid becoming untidy and messy. Oil also can leak from the housing over time. Additionally, the oil can become contaminated, or degraded due to temperature and pressure fluctuations associated with the operation of the vacuum tube. As a result, risk of failures from high voltage arcing, leakage and breakdown tend to increase over time for the liquid oil-based systems.
Therefore, as an alternative to oil, a silicone rubber resin has been used for potting the housing with an internally mounted vacuum tube. Namely, room temperature vulcanizing (RTV) silicone rubber has been used for vacuum x-ray tube assemblies to displace all the air otherwise present in the gap provided between a vacuum tube and its housing. A curable liquid RTV silicone rubber is poured in a liquid state inside the housing to directly contact its inner walls. The vacuum tube is submerged in the silicone rubber such that the silicone rubber completely fills the gap between the exterior surface of the vacuum tube and the inner walls of the housing. Once the RTV rubber is cured, the housing, vacuum tube and intervening silicon rubber form a unitary assembly. Once the RTV rubber is cured, such housing systems are often very stable with adequate insulation and thermal conduction properties. The cured silicone rubber does not leak from the housing. Like oil systems, silicone rubber systems generally are reliable for many implementations.
However, a drawback to using potting silicone resins in the above-stated conventional manner derives from the fact that a defective vacuum tube cannot be serviced or replaced without considerable difficulty since the cured potting material is adhesively bonded to not only the vacuum tube but also the housing, and thus it is not easily detached and removed from the housing.
It would be desirable to be able to provide a vacuum tube housing system that has all of the advantages of silicone rubber potting systems while permitting easier, more convenient removal and/or replacement of the vacuum tube from a housing.
According to the present invention, a high voltage device housing assembly includes a housing and a high voltage device assembly arranged in combination with an improved insulation system which effectively prevents high voltage breakdown while permitting easy and convenient installation, removal and/or replacement of the high voltage device from its housing.
In one embodiment, a vacuum tube housing assembly of this invention includes a housing having an inner wall defining an enclosure into which a vacuum tube assembly is inserted. The vacuum tube assembly includes a vacuum tube having a outer side surface bearing an insulator, where the insulator includes a first portion generally continuously covering the outside surface of the vacuum tube and an integral second portion comprising a plurality of spaced apart projections or ribs extending around the outside surface and between the first portion and the inner wall of the housing. Air gaps are present between the respective projections or ribs, and the spacing of the ribs is established in a manner that inhibits high voltage breakdowns from occurring between the interface of the housing and the vacuum tube.
In one advantageous embodiment, the insulator used in the inventive vacuum tube housing assembly is formed from a curable silicon rubber molded in situ upon the exterior surface of the vacuum tube. The silicone rubber is molded and cured in place in a manner providing a series of spaced apart, raised circular ribs in the resulting rubber encapsulant formed upon the vacuum tube surface. This ribbed configuration of the rubber encapsulant formed upon the vacuum tube permits the resulting pre-potted vacuum tube to be properly positioned within the housing such that the spaced apart ribs physically engage the inner wall of the housing to form seal zones that alternate with intervening air gaps created between adjoining ribs and the inner wall of the housing. The resulting silicone rubber pre-potted vacuum tube assembly is endowed with high voltage insulation for high voltage connections within the housing. Moreover, the inventive pre-potted vacuum tube structure can be easily plugged into its housing and just as easily removed, if necessary, because the rubber encapsulant is not adhered to the housing, thereby greatly improving the manufacturability and serviceability of housed vacuum tubes. Furthermore, the rib features of the rubber encapsulant also serve to thermally couple heat from the vacuum tube to the housing.
Although the present invention is especially well-suited for implementation in vacuum x-ray tube assemblies, it is by no means limited thereto and has general applicability to housed high voltage components, including, for instance, high voltage resistors and high voltage capacitors. The present invention also is directed to a method of making the high voltage device housing assembly.